Analog signal processing is commonplace in a wide variety of low-power, low-cost applications such as mobile telephones, laptops, personal digital assistants (PDAs), hearing aids, etc. These applications usually require low noise, low power, and high performance circuits. In addition, these applications require low total harmonic distortion (THD) which is related to the linearity of the circuit. To achieve the desired operating characteristics, many of these signal processing applications use programmable gain amplifiers (PGAs) and analog filters where signals of varying strengths must be either amplified or attenuated before signal processing.
A key component of several analog signal processing circuits is a linear transconductance cell. For example, linear transconductance cells are used in analog filters and PGAs. A common implementation of a linear transconductance cells uses a resistively degenerated differential transistor pair with one or more resistors placed in series between the source nodes of the two transistors. The differential transistor pair is operably coupled to receive a differential input voltage and, based on the transconductance, produce a differential output current. However, in these differential transistor pairs, the transistors effect the transconductance. As a result, the linearity of the analog signal processing circuit is limited. Furthermore, the value of the transconductance depends on the transistor characteristics, which vary as a function of the manufacture process and the temperature of the device. Consequently, the gain of the transconductance cell is not accurate.
Resistive degeneration is typically used in the differential transistor pair to reduce the effect of the transistors on the transconductance value. For example, if the conductance of one or more resistors placed in series between the source terminals of the differential pair transistors is much smaller than the transconductance of the transistors, the overall transconductance is substantially determined by the conductance of the resistors. Consequently, the transistors have less effect on the transconductance and the linearity of the circuit is improved. Furthermore, the transconductance value depends primarily on resistor conductance, which typically depends less upon the manufacture process and temperature. However, these improvements in linearity and transconductance are expensive. A differential transistor pair implementation without resistive degeneration typically requires less power and area to obtain a specified transconductance value with a specified level of current noise.
Therefore, a need exists for a transconductance cell that has low noise, good linearity, good gain accuracy, low power consumption, and small area.